1. Field of the Invention
This invention relates to a bladder mandrel which may be used to expand tubes and sleeves in steam generators. The invention is particularly useful in creating interference-type joints between reinforcing sleeves and the heat exchange tubes in nuclear steam generators.
2. Description of the Prior Art
Mandrels for hydraulically expanding the heat exchange tubes in nuclear steam generators are known in the prior art. Such mandrels typically include a mandrel body having an orifice for conducting pressurized water from a hydraulic expansion unit to the outer surface of the mandrel, and a pair of O-rings which flank the orifice on either side. The O-rings circumscribe a pair of opposing, frustro-conical ramps which extend away from the fluid orifice in both directions. When the mandrel is inserted within the tube (or sleeve) to be expanded, the outer edges of the resilient O-rings come into sealing engagement with the inner walls of the tube. When pressurized water is pumped through the fluid orifice, the annular space between the two O-rings which flanks the orifice fills with pressurized water, which in turn rolls each of the O-rings up its respective frustro-conical ramp and wedges it progressively tighter between the body of the mandrel and the inner walls of the tube. Finally, each of the O-rings rolls into a spring-loaded shoulder which circumscribes the distal end of its respective frustro-conical ramp, which arrests the motion of the O-ring. The hydraulic pressure exerted by the water then causes the region of the tube between the O-rings to expand outwardly until it is inelastically deformed about its radius. A more complete description of both the structure and operation of such mandrels is set forth in U.S. patent application Ser. No. 567,107, filed Dec. 30, 1983 and assigned to Westinghouse Electric Corporation, the entire specification of which is expressly incorporated herein by reference.
To fully appreciate the importance of the radial tube and sleeve expansions implemented by such mandrels, some background in the maintenance problems and procedures associated with steam generators is necessary.
Nuclear steam generators generally include a primary side through which hot, radioactive water from the reactor core is admitted into a plurality of U-shaped heat exchange tubes. Such steam generators further include a secondary side which houses and spaces these tubes and circulates a flow of non-radioactive water therethrough, so that non-radioactive steam may be generated from the energy output of the reactor core. The primary and secondary sides of the steam generator are separated by means of a tubesheet having an array of bores, in which both the inlets and outlets of the U-shaped heat exchange tubes are mounted. In the secondary side of the steam generator, the U-shaped tubes are uniformly spaced apart from one another by an array of horizontally disposed support plates, each of which includes a plurality of bores through which the U-shaped tubes extend.
Despite the fact that both the tubesheet and the U-shaped tubes are formed from corrosion-resistant metals such as Inconel, a serious amount of tube corrosion has occurred in many nuclear steam generators in the sections of the U-shaped tubes which are mounted within the bores in the tubesheet, and those sections of the U-shaped tubes which extend through the bores in the support plates. More specifically, corrosive sludges have been found to accumulate in the annular spaces between the U-shaped tubes and the bores in both the tubesheets and the support plates. Additionally, the water currents generated by the inflow of non-radioactive water in the secondary side of the generator sometimes causes the heat exchange tubes in the secondary side to vibrate within the annular space between the outside surface of the tube, and the outside surface of the bores in the support plates through which the tubes extend. These corrosive sludges and mechanical vibrations can degrade the integrity of the tube walls in these regions until the walls of these tubes crack and leak. When this happens, radioactive water from the primary side of the nuclear steam generator contaminates the non-radioactive water flowing through the secondary side of the generator.
Hydraulic expansion mandrels are useful in repairing the sections of the U-shaped tubes which have been degraded by such corrosion. In the tubesheet region of the generator, such a mandrel is slid up to the region of the tube in need of repair, along with a reinforcing sleeve (which is usually a tubular section of Inconel having an outer diameter slightly smaller than the inner diameter of the tube). Once the reinforcing sleeve is properly positioned across the degraded section of the tube in need of repair, the mandrel is actuated. Sleeving mandresl typically include two sets of O-rings, and the pressurized water which accumulates between the two sets of O-rings radially expands the reinforcing sleeve at both its top and bottom portions until it is inelastically deformed into the walls of the heat exchange tube in an interference-type joint. In the sections of the tubes which extend through the bores in the support plates, such mandrels may be used to expand the heat exchange tubes so that essentially all of the annular clearance between the tube and the walls of the bore of the support plate is eliminated, which keeps the tube from rattling within the bore and minimizes the possibility of corrosive sludge accumulating between the tube and the bore.
While such mandrels have been used successfully in many tube repairs in nuclear steam generators, certain problems remain. For example, mandrels utilizing a dual O-ring configuration require the use of lubricants such as glycerine on the inner walls of the tube or sleeve prior to insertion of the mandrel within the tube. If such lubricants are not used, the O-rings will tend to bind between the mandrel body and the inner walls of the heat exchange tube when they are being slid up to the section of the tube in need of repair. Such binding will tend to wear out the elastomeric substance forming the O-rings, which in turn will jeopardize the integrity of the water-tight seal when pressurized water wedges the O-rings between the inner wall of the tube (or sleeve) and their respective frustro-conical ramps on the body of the mandrel. Unfortunately, the use of such lubricants to solve the binding problem not only prolongs the amount of time that a worker must be exposed to radiation present in the primary side of the nuclear steam generator, but also creates problems if one wishes to augment the interference-type joint between the sleeve and the tube with a braze joint. Specifically, the glycerine present along the inside walls of the tube can vaporize just as the brazing alloy begins to set, thereby jeopardizing the integrity of the seal between the hardening brazing alloy and the tube in the sleeve. In addition to the need for lubricants, still another problem associated with such prior art mandrels is the limited amount of diametrical expansion which they can impart on the tube or the sleeve-tube combination. Mandrels employing O-rings typically have a maximum diametrical expansion capability of only 0.045 in., at which point the pressurized water begins to extrude the O-rings between the inner walls of the tube or sleeve and the spring-loaded retaining shoulders located on the ends of the frustro-conical ramps. thereby damaging them. Such O-ring damage will adversely affect the reliability of the mandrel in producing expansions, and will shorten the life of the O-rings, which in turn will necessitate more frequent replacement of these rings.
Clearly, there is a need for a hydraulic expansion mandrel which is capable of being easily inserted into a sleeve or a tube without the need for lubricants, and which has the capability of expanding the tubes or the sleeves by large diametrical amounts with no loss in reliability. Further, it would be desirable if the sealing mechanism of this mandrel were capable of effecting a great number of expansions before replacement of the sealing parts became necessary.